DE2737530A1 - Channel filter for FM communications signal - has transfer characteristic which minimises differential gain and phase spreads for given out of band attenuation - Google Patents

Abstract

The channel filter, for FM or angle-modulated communications signals in which the baseband signal includes one or more subcarriers, has a steeper rise in attenuation for constant distortion or fewer circuit components for the same rise in attenuation. The filter's transfer function is chosen so that the spread of the differential gain and/or the differential phase at a given attenuation outside the band is a minimum. The filter is thereby optimised directly for minimum differential distortion in the video band and can consequently be used for higher quality tv transmission without having to increase the number of components.

Description

Filter circuit for high-frequency electrical communications

The invention relates to a filter circuit for high frequency electrical communication signals generated by frequency or angle modulation of a Baseband signals have arisen, in particular a baseband signal with subcarriers, wherein the filter circuit has the transfer function H (f).

It is well known that transmission systems with angle modulation (frequency iodulation) insensitive to non-linear distortions in the HF path (RF path). Against it can use linear components with linear distortion in the RF link in the baseband cause non-linear distortion, which makes the transmission quality decisive can be worsened. Linear distortions can be in the form of a transfer function H (f) can be specified as the amplitude and transit time response.

Another way to characterize the linear RF distortion is the indication of their effects in the baseband in the form of differential phase and differential gain. These two measurable quantities play a particular role Transmission systems play a role in which one or more Subcarrier be transmitted. For example, are differential phase and differential gain representative standards for the transmission quality of a closed frequency modulator th color television picture with one or more sound subcarriers.

Details on this can be found in the references "ABU 27 "(1973), pages 98-100 and pages 406-407.

If the filters are sized in the manner specified there, then the following difficulties arise in practical operation.

With large values for differential phase to 20 °) and differential Gain to 20X) the luminance signal modulates the color subcarrier in such a way that a gray wedge after the transmission line appears with brightly colored stripes. It also occurs in the sound channel due to differential phase and amplitude, the luminance signal appears as an interference signal.

Investigations on which the invention is based have shown the following. Linear distortions in the RF link are mainly caused by amplitude and Runtime response caused by filters used in transmission systems for channel selection are necessary. Predefined values for adjacent channel suppression determine here and bounds for the differential distortions the filter effort. At the one today The usual design of the transmission path is the filter of one of the known ones Types, e.g. a filter with Chebyshev behavior. These filters are optional with regard to their distortions in the passband, but not in the case of a transmission of frequency or angle modulated signals with respect to differential distortion in the baseband.

The invention is based on the object of filter circuits for Specify transmission of frequency or angle-modulated message signals with which with the same distortion a steeper increase in attenuation can be achieved or which have a lower number of switching elements with the same increase in locking attenuation require known filters of the corresponding design.

Based on the filters mentioned in the introduction, this task is solved according to the invention in that the transfer function H (f) is chosen in such a way is that the fluctuation range of the differential gain and / or the differential Phase with a given outer band attenuation becomes a minimum.

Such filters are therefore direct for minimal differential distortion Optimized in the videotape and it can therefore be a substantial effort with the same amount of effort improved TV transmission can be achieved.

The invention is explained in more detail below.

Equation (1) and equation (2) describe in a quasi-stationary approximation the relationship between differential phaso ## and differential gain # and the transfer function H (f) of the filter.

Here, kp mean the AM-PM conversion coefficient of a dol filter A switched non-linear quadrupole, e.g. a traveling wave tube, is the current one Frequency offset caused by the baseband sum signal and fs the split frequency, for example the frequency of a subcarrier. H (f) is the transfer function of the filter, defined as the quotient of the complex output voltage to half the generator voltage in adapted operation. H * (f) represents the conjugate complex value for H (f).

In contrast to the previously common filter diminution, now dX Transfer function H (f) in equation (1) and equation (2) equaled such that one of the fluctuation ranges or both fluctuation ranges of the differential distortions # and ## assume a minimum for a given out-of-band attenuation. This can be done on can be solved in various ways. For example, it is possible for the differential distortions have a Chebyshev polynomial curve over the Apply frequency offset fa and a Hurwitz polynomial in the denominator of the transfer function H (f), whose coefficients with the method of coefficient comparison in any high approximation can be determined depending on the computational effort.

For the special case that only the differential gain is to be optimized, the coefficients ar of the absolute value square of the transfer function H (f) can be determined from the following equation for the so-called equivalent low-pass filter of degree n: where t # are the coefficients of the Chebyshev polynomial (2n + 2) of the order and y = 2fs / # fss. The peak-to-peak frequency deviation #fss and the slit frequency fs can be freely specified.

In principle, the optimization of equations (1) can be done with (2) can also be carried out purely numerically with the aid of a digital computer.

The method of optimization for minimal differential distortion is not tied to specific circuit structures. Rather, it depends on the desired Circuit structure the form for the approach for the transmission function H (f), according to which is then also the switching elements in a manner known per se.

have the values determined for the filter. For example, after the Book by G. Pfitzenmaier (Book of tables low passes "(Siemens AG, Berlin and Munich, 1971) the above-mentioned equivalent animal pass circuit based on the provided Requirements are determined and from this through one of the frequency transformations also specified there a suitable bandpass circuit has been determined.

To illustrate the profit that can be achieved, for an FM color television transmission system (with emphasis) the out-of-band attenuation with a given fluctuation range of the differential Determined profit for the color subcarrier. The results are for Chebyshev Filters and for filters optimized according to the above explanations in the following Table juxtaposed.

Number of circles 2 3 4 5 6 Out-of-band attenuation 22.7 32.1 44.5 60 76 dB Cheby ship out-of-band attenuation 22.7 51.7> 80 - - dB differential gain optimized The table above shows a comparison of Chebyshev bandpasses with those which are optimized with regard to the differential gain of the color subcarrier. Of the differential gain for all filters is 1% for the filter carrier. Have to the bandpass filters meet the following requirements: bandwidth 27MHz, out-of-band attenuation at 46.5MHz from channel mean, emphasis according to CCIR Rec. 405-1. Peak deviation at 15.6KHz, 4.36MHz with a video signal of IV.

Assuming filters with the same out-of-band attenuation, it becomes apparent that that an optimized four-circuit filter is even less by a factor of 3.1 has differential gain than a six-circle Chebyshev filter.

With the filter circuits described above, a low-distortion is achieved Angle or frequency modulated transmission of baseband aials with subcarriers with simultaneous high channel selection. These filters are not as usual, dimensioned for minimal distortion in the passband, but directly for minimal differential distortion in the baseband. This creates a type of filter that, for example the requirements given in the table above with a reduced number of circles Fulfills. This creates a type of filter for frequency-modulated transmission systems, the minimum differential at the same time with high out-of-band attenuation in the RF channel Has amplitude distortions of subcarriers in the baseband.

A quasi-stationary approximation can be used for the equivalent low-pass filter first determine the optimal transfer function analytically. Based on the specified Dimensioning, the coefficients of the transfer function H (f) can then be determined.

The comparison with conventional filter types shows that with the same Component expenditure a considerable reduction in the differential amplitude distortion is possible, for example for the color or sound subcarrier of a TV signal.

1 claim

Claims (1)

Patent claim filter circuit for high-frequency electrical communication signals, caused by frequency or angle modulation of a baseband signal, in particular a baseband signal with subcarriers, the filter circuit the transfer fraction H (f) has, d u r c h g e k e n n n -z e i c h n e t, that the transfer function H (f) is chosen such that the fluctuation range the differential gain (b) and / or the differential phase (b) at a given Outer band steaming becomes a minimum.